An artist's impression of the Ulysses spacecraft passing through the tail of comet Hyakutake.

Ulysses unexpectedly encountered the tail of comet Hyakutake in May 1996. At the time, Hyakutake's nucleus was close to the Sun - more than 525,000,000 km (326,000,000 miles) away. The measurement was the longest comet tail ever recorded.

The discovery revealed comet tails - streams of ions, gas and dust extending away from the Sun - were much longer than previously believed.